1. Technical Field of the Invention
The present invention relates to a technique of reforming amorphous semiconductor film such as a silicon film into a polycrystalline or monocrystalline semiconductor film by irradiating a rectangular laser beam onto the amorphous semiconductor film on a substrate in fabricating a semiconductor device, and a technique of improving the quality of a polycrystalline or monocrystalline semiconductor film by irradiating a rectangular laser beam onto the polycrystalline or monocrystalline semiconductor film on a substrate. As an original polycrystalline or monocrystalline semiconductor film whose quality is to be improved, there is a film prepared by solid-phase growth or a film prepared by laser annealing. Improvement of the quality of a polycrystalline or monocrystalline semiconductor film means (1) increasing the size of crystal grains, (2) decreasing defects in crystal grains, and (3) crystallization of an amorphous portion remaining among crystal grains.
2. Description of the Related Art
In a case where a thin film transistor (hereinafter called “TFT”) is formed on a substrate in fabrication of a semiconductor device, the use of an amorphous semiconductor film, such as an amorphous silicon film, as a semiconductor layer where TFTs are to be formed cannot achieve a fast operation due to a lower mobility of carriers. In this respect, an amorphous silicon film is usually transformed into a polycrystalline or monocrystalline silicon film crystallized by laser annealing.
To transform an amorphous silicon film into a polycrystalline or monocrystalline silicon film by laser annealing, a laser beam whose cross section perpendicular to the advancing direction is a rectangle (hereinafter called “rectangular laser beam”) is often used. A rectangular laser beam is irradiated on an amorphous silicon film while moving the substrate having the amorphous silicon film formed thereon in a short-side direction of the rectangle. A method of forming a polycrystalline or monocrystalline silicon film with a rectangular laser beam is disclosed, in Patent Document 1 described below, for example.
Non-patent Documents 2 and 3 described below show techniques relevant to the present invention. Those documents describe that when a polarized laser beam is irradiated onto a solid surface, a surface electromagnetic wave is excited on the solid surface and interference of the surface electromagnetic wave with the incident laser beam generates a standing wave on the solid surface, thereby forming a micro periodic structure on the solid surface.
[Patent Document 1]
Japanese Laid-Open Patent Publication No. 2003-347210 “SEMICONDUCTOR DEVICE AND FABRICATION METHOD THEREFOR”
[Non-patent Document 1]
www.nml.co.jp/new-business/SUB2/investigation/ripples/texture.pdf
[Non-patent Document 2]
Laser Study December 2000, Vol. 28, No. 12, pp. 824-828 “Incident-Angle Dependency of Laser-induced Surface Ripples on Metals and Semiconductors”
[Non-patent Document 3]
pp. 1384-1401, IEE JOURNAL OF QUANTUM ELECTRONICS. VOL. QE-22, NO 8, August, 1986
In a process of forming polycrystalline or monocrystalline silicon by irradiation of a rectangular laser beam, the direction of growth of crystal grains is greatly affected by temperature gradient or energy gradient of the laser beam. As shown in FIGS. 1A, 1B and 1C, the energy of the rectangular laser beam in the long-side direction is constant, so that a nucleus is generated at a random position relative to the long-side direction. This results in growth of the nucleus to a random size.
The energy distribution of rectangular laser beam in the short-side direction has a large gradient as shown in FIG. 2. Because the crystal growth is extremely sensitive to the energy distribution in the short-Side direction, therefore, it is very difficult to make the crystal size in the short-side direction uniform. As a result, a variation in crystal size in the short-side direction becomes greater than a variation in crystal size in the long-side direction as shown in FIG. 3.
As apparent from the above, conventionally, a polycrystalline or monocrystalline silicon film having crystal grains with an nonuniform size is formed.
Accordingly, when TFTs are formed on the polycrystalline or monocrystalline silicon film, the performance of the TFTs varies due to a difference in the number of crystal grains in the channel portion per unit length. As the size of crystal grains greatly differs between the short-side direction and the long-side direction, the performance of TFTs greatly differs between the short-side direction and the long-side direction. This is because the performance of TFTs becomes lower by increase in the number of times a carrier moving the channel portion encounter the crystal grain boundary.